FIG. 2 shows an example of the configuration of an ultrasonic diagnostic apparatus according to prior art using a digital circuit. An ultrasonic signal s (t) sent from a transducer 1 composed of plural elements and having a mean frequency ωs can be approximately expressed by an expression 1. “A (t)” denotes the shape of an envelope of the sent signal, “t” denotes a variable of time and “j” denotes an imaginary unit.s(t)=A(t)×{exp(jωst)+exp(−jωst)}  Expression 1
The transducer 1 receives an ultrasonic wave reflected on an object of inspection. A signal fn (t) received by an “n”th element of the transducer 1 is expressed by an expression 2 in case propagation delay time from the sending of an ultrasonic wave to the receiving of it is τn. “φn” denotes a phase of the received signal fn (t) and is acquired in an expression 3.                                                                                                                        f                    n                                    ⁡                                      (                    t                    )                                                  =                                ⁢                                  s                  ⁡                                      (                                          t                      -                                              τ                        n                                                              )                                                                                                                          =                                ⁢                                                      A                    ⁡                                          (                                              t                        -                                                  τ                          n                                                                    )                                                        ×                                      {                                                                  exp                        ⁡                                                  [                                                      j                            ⁡                                                          (                                                                                                                                    ω                                    s                                                                    ⁢                                  t                                                                -                                                                  ϕ                                  n                                                                                            )                                                                                ]                                                                    +                                              exp                        ⁡                                                  [                                                      -                                                          j                              ⁡                                                              (                                                                                                                                            ω                                      s                                                                        ⁢                                    t                                                                    -                                                                      ϕ                                    n                                                                                                  )                                                                                                              ]                                                                                      }                                                                                                          Expression        ⁢                                   ⁢        2             φn=ωsτn  Expression 3
When the number of elements used for one sending/receiving of an ultrasonic wave is N, N pieces of output signals from the transducer 1 are respectively expressed by the expression 2. The propagation delay time τn is different for every element. In the following description, element n used for a series of one sending/receiving of an ultrasonic wave are called channel n and “N” denotes the number of channels. In FIGS. 1, 2 and 6, a selecting circuit and a driving circuit for N elements used for a series of sending/receiving of an ultrasonic wave are omitted and are not shown.
An analog to digital converter 2 converts the received signal fn (t) to digital data and the succeeding signal processing is all digital signal processing. The precision of operation is enhanced by digital signal processing, compared with that in analog signal processing. An A/D converter is generally used for the analog to digital conversion part 2.
A mixer 3 multiplies the received signal fn (t) converted to digital data and a digital reference signal hn (t) expressed by an expression 4. A product gn (t) of multiplication is expressed by an expression 5. “hn (t)” has the same frequency as the center frequency ωs of the received signal.                                           h            n                    ⁡                      (            t            )                          =                  exp          ⁡                      (                                          jω                s                            ⁢              t                        )                                              Expression        ⁢                                   ⁢        4                                                                                                      g                  n                                ⁡                                  (                  t                  )                                            =                                                                    f                    n                                    ⁡                                      (                    t                    )                                                  ⁢                                                      h                    n                                    ⁡                                      (                    t                    )                                                                                                                          =                                                A                  ⁡                                      (                                          t                      -                                              τ                        n                                                              )                                                  ×                                  {                                                            exp                      ⁡                                              [                                                  j                          ⁡                                                      (                                                                                          2                                ⁢                                                                  ω                                  s                                                                ⁢                                t                                                            -                                                              ϕ                                n                                                                                      )                                                                          ]                                                              +                                          exp                      ⁡                                              (                                                  j                          ⁢                                                                                                           ⁢                                                      ϕ                            n                                                                          )                                                                                                                                                    Expression        ⁢                                   ⁢        5            
Next, a filter 4 extracts a low-frequency component from the product of multiplication (the expression 5). The product of the multiplication from which the low-frequency component is extracted is expressed by an expression 6. The filter 4 is formed by an accumulator and an element for computing the sum of products for example.gn(t)=A(t−τn)×exp(jφn)  Expression 6
A digital delay unit 5 multiplies a signal acquired by delaying a signal output from the filter 4 and expressed by the expression 6 by τn by exp(−jφn). A signal Vn (t) output from the digital delay unit 5 is expressed by an expression 7. The output signal Vn (t) is fixed independently of a channel without depending upon n.                                                                                                                  V                    n                                    ⁡                                      (                    t                    )                                                  =                                                                            g                      n                                        ⁡                                          (                                              t                        +                                                  τ                          n                                                                    )                                                        ×                                      exp                    ⁡                                          (                                                                        -                          j                                                ⁢                                                                                                   ⁢                                                  ϕ                          n                                                                    )                                                                                                                                              =                                  A                  ⁡                                      (                    t                    )                                                                                                            Expression          ⁢                                           ⁢          7                    
The output signal Vn (t) output from the digital delay unit 5 is added in an adder 6 by the number (N) of all channels of a series of element n (channel n) used for a series of sending/receiving of an ultrasonic wave. A result of addition grows N times of a signal of a single channel, if a phase of each channel is coincident. N signal lines from the transducer 1 to the adder 6 are converted to a signal line in the adder 6.
In the above description, as each processing in the analog to digital converter 2, the mixer 3, the filter 4 and the digital delay unit 5 is executed every channel, parallel N pieces of respective units are required.
In this process, signals received from directions except a desired direction vanish because they have different phases. Signal processing described above is generally called beam forming. Ultrasonic beams can be formed in the desired direction by the beam forming.
For documents related to the beam forming addition, there are Japanese patent No. 1333370 and U.S. Pat. No. 4,140,022 and U.S. Pat. No. 4,983,970.
In an envelope detector 7, the absolute value of “N×A (t)” which is a signal output from the adder 6 is acquired and a scan converter 8 applies signal processing such as the compression of a logarithm and gamma conversion to a signal output from the envelope detector 7. A signal output from the scan converter 8 is displayed on a display 9 as a tomographic image of an object to be inspected. In the expression 7, a complex number is generally acquired and the envelope detector 7 calculates the absolute value of the complex number (the root square sum of the real part and the imaginary part).
In the ultrasonic diagnostic apparatus according to the prior art shown in FIG. 2, the center frequency of the received signal to be imaged is required to be determined beforehand. In the conventional type apparatus shown in FIG. 2, the center frequency of the received signal to be imaged is the same ωs as the center frequency of the sent signal and is equal to the frequency of the digital reference signal hn (t). The ultrasonic diagnostic apparatus according to the prior art shown in FIG. 2 has the problem that the frequency of the digital reference signal hn (t) is required to be equalized to the center frequency of the received signal to be imaged and the center frequency of the received signal to be imaged is limited to the predetermined one frequency.
The ultrasonic diagnostic apparatus according to the prior art shown in FIG. 2 has the problem that as the filter 4 removes the high-frequency component (2ωs) which is an unnecessary component from the signal output from the mixer 3, the frequency of the unnecessary high-frequency component also varies in case the center frequency ωs of the received signal to be imaged is varied and the pass band of the filter 4 is required to be varied in accordance with the center frequency of the received signal to be imaged.
There is the problem that as the filter 4 is required for every channel, the scale of the apparatus is enlarged when the configuration of the filter 4 is complex and the apparatus becomes high-priced.